For everyday use I think the nod goes to the latest, or at least a very recent, stable/official release. In part because there should be fewer bugs and unrefined features. In part because other channels are more likely to have [additional] telemetry, experiments, etc enabled by default. Some pre-release software, possibly even browser software, may not even have or respond to opt-out preferences for such features. Plus, if you are using security/privacy related extensions, those may not yet be equipped to properly deal with the behavior of pre-release software.

I think the guinnea pig versions are quite useful for testing purposes. You can mitigate some issues by restricting their communications to localhost or your local network.

I'm staying with an older version. The newer versions are becoming more bloated with each release. Most of their new features I don't need or want. Too many of these features include behaviors that are hostile to a users privacy, unique identifiers, calling home, and infested with Google garbage. IMO, any security improvements they contain are more than offset by the privacy implications of these new features. I won't trade away privacy for security, especially when I can mitigate most of the security issues with Proxomitron. I'm also tired of the broken extension game these constant update policies create.

The whole beta/alpha-testing is aimed at those who want to see if there's any looked over issues with their software before they pump it out to the masses. If users have the time and want to do some unpaid bug testing by using it, then it could save other users a lot of headache when it comes time for that version to be released as stable (like catching a bug that might not had been caught). So when I try to submit bugs for any software, I do it mainly motivated by the idea of saving another user the trouble, and hopefully help get a fix out. Web browsers though... I've found honestly there's enough issues with stable without getting into all the separate plugin and extension issues.

Basically, less they finally pump out an official 64bit version of Firefox for Windows, I'm alright with stable. (and yes I've tried Waterfox)

Stable, but an old version. FF v27.0.1 to be precise... because I do not trust it anymore, nor like the changes made to it since. Don't know what I'll migrate to if/when there's something that affects me making this version no longer safe for me to use, because I trust Chrome even less and just despise IE.